It is commonplace for manufacturers to provide card racks or enclosures for receiving and supporting circuit cards (sometimes referred to as circuit boards, printed circuit boards (PCBs), or electronic circuit boards (ECBs)). Typically the cards are inserted into slots by sliding them along a card guide which aligns the card edge connector with its mating connector mounted at the back of the enclosure.
The cards have electronic components mounted thereon for performing specific tasks, such as data acquisition, measurement, or signal amplification, for example. Due to normal operation, these cards dissipate power in the form of heat In some cases the amount of heat given off by the cards is very low, and dissipates naturally into the surrounding air. However, in many cases the heat dissipation is of such an extent that the cards need to be cooled by airflow from a fan. Some early prior art card racks installed the fan at one end and the air exhaust ports at the other end, in the hope that the air would circulate around the circuit board components and carry off the heat.
Later prior art card racks, in an effort to better direct the airflow to the circuit cards, mounted the fan at the back of the enclosure to pressurize a plenum which had ports beneath the circuit cards. In this way, the air was blown directly up along the faces of the circuit card. Unfortunately, air was not only directed to occupied slots where it was needed, but also, needlessly, to unoccupied slots. Some method was needed to block the airflow to unoccupied slots, because the air delivered to the unoccupied slots was "wasted" in that it reduced the airflow to the occupied slots. Prior systems solved the problem by filling the otherwise unoccupied slots with dummy loads designed to impede the airflow to those slots. The solution worked reasonably well, but raised its own problem. The dummy loads were temporary fillers only used when a circuit board was not installed. Often they would be replaced in the slot by a working circuit board, and promptly missed. When the need for that particular working circuit board ended, the user would remove it. If, However, the dummy load is mislaid, it cannot be reinserted, which will cause airflow to be, once again, directed to an empty slot.
U.S. Pat. No. 5,528,455 (Miles) MODULAR INSTRUMENT CHASSIS, issued Jun. 18, 1996, and assigned to Tektronix, Inc. Wilsonville, Oreg., describes a ventilating system for circuit card guides which employs movable dampers to automatically direct airflow to occupied slots and impede airflow to unoccupied slots. In this system, the dampers included a plurality of holes which are moved into alignment with a similar plurality of holes formed in the roof plate of an air plenum located beneath the circuit card guides. Thus, insertion of the card causes air to begin flowing to the circuit card. While this system works well, it is limited in three important ways. First, the dampers, when fully opened, block fifty percent of the airway. Second, the dampers when fully closed leak at each seal. Unfortunately, the linear seal length of this system is quite long in that it comprises the sum of the perimeters of each hole in the plenum roof plate. Third, the manufacturing cost and assembly time are significant due to the complexity of the system.